My 87 Iroc-Z keeps shifting back and forth from drive to overdrive while driving at low speed, about 20-40 mph. It dosen't do it at faster speeds. If I shift it manually through the gears it doesn't do it. Does anyone have an idea of what might be causing this.

 

I've got the same problem with my 83... in fact every gm I've had with the 700R4 has done that...
let me know what you find out...
its more pronounced in my 83 with the shift kit then the others were...

 

My first guess would be that your TV cable needs adjustment / replacement.

http://wiki.answers.com/Q/What_is_th...evy_Camaro_Z28

Probably explains why it works when you manually shift it... I got that problem with my cavalier... It's annoying as hell i feel ur pain lol..

Later

 

What does it look like? How do you go about adjusting it, and how much?

 

Really only one way to reset the TV cable. Not intended to be 'adjustable'. But it does sound like a low line pressure rise, so readjust it as instructed and try it out.
But that issue is an inherent problem and a shift correction kit from Superior or ProBuilt should fix the issue permanently